Diagnostic circuit for a supplemental inflatable restraint system

Abstract: An inflatable restraint system includes a diagnostic circuit (18) which measures the resistance of an external deployment path (12) containing a squib. The circuit (18) supplies a test current through the path (12) and measures the voltage across the path which is monitored by a microprocessor (36). A vehicle battery (22) supplies the power to the system and is connected to a constant current source (26) in series with the external path (12) to provide the test current. A current limiting device (42) between the external circuit and ground comprises a current sink (42) allowing a maximum safe current in the external circuit in the event of inadvertent application of battery voltage to the external circuit (12) and, during a normal resistance test, imposes only a small voltage drop on the test current circuit (18).

Description: This invention relates to a diagnostic circuit for a supplemental inflatable restraint system. Supplemental inflatable restraints (SIR), commonly known as air bags, are used in automotive vehicles to cushion the driver or other occupant from injury during an impact. Such systems include a sensing and deployment module, a squib controlled by the module to fire when an impact is sensed, an inflatable bag, and an inflating device triggered by the squib to inflate the bag. The squib is outside the module in an external deployment path. The module includes a diagnostic capability for checking or monitoring the resistance of the external path, thereby verifying that the squib is operational. A conventional diagnostic circuit for measuring the resistance in the external path of a SIR is shown in Figure 1. The external path 12 containing the squib 14 is coupled by connectors 16 to the sensing and deployment module 18 which includes the diagnostic circuit 20. The diagnostic circuit is supplied by the vehicle battery 22 and the voltage is increased and regulated by a boost supply 24. A constant current source 26 is connected from the boost supply 24 to one end of the external path via a connector 16, while the other connector 16 connects the path 12 through a current limiting device comprising resistor 28 and a transistor switch 30 to ground. The inputs of a differential amplifier 32 are connected to either end of the external path 12 adjacent the connectors 16 and the amplifier output is fed through an A/D converter 34 and the resulting digital data is fed to a microprocessor 36. The switch 30 is controlled directly or indirectly by the microprocessor 36 via line 38. The circuit operates when the transistor switch 30 is turned on, allowing the current output of the constant current source 26 to pass through the external path 12 to ground. The voltage developed across the external path, which is indicative of the integrity or fitness of the path, is amplified and sampled by the microprocessor for analysis. The current of the current source 26 is chosen to be small enough, for example 150 mA, to impose no danger of either firing the squib or heating it enough to cause chemical deterioration. When the switch 30 is turned on for a test, the fixed current flows through the external path 12 and the voltage generated across the path is amplified, digitised and then assessed by the microprocessor which is programmed to determine the condition of the external path. The resistor 28 is of the order of 100 ohms and is necessary to limit current to a harmless value in the event that battery voltage is inadvertently applied to the high end of the external path 12 while the transistor switch 30 is conducting. The test current of 150 mA will cause a voltage drop of 15 volts in the resistor 28 and additional voltage drops in current source 26, the squib 14 and the transistor 30 for a total voltage drop of at least 18 volts. The vehicle battery voltage, having a range of 9 to 16 volts is insufficient to supply 18 volts directly so that the boost supply 24, generating 18 to 36 volts, is necessary. US-A-5155376 discloses a circuit which is equivalent to the above described arrangement. WO-A-88/01241 describes a driving and monitoring element for an ignition circuit for a vehicle safety device which uses a high voltage source, a low-side current sink and a comparator. The present invention seeks to provide an improved diagnostic circuit. A diagnostic circuit in accordance with the present invention is characterised over US-A-5155376 by the features specified in the characterising portion of claim 1. It is possible with the invention to avoid the expense of a boost supply. It is also possible to provide a circuit which can test the external deployment path of a supplemental inflatable restraint using vehicle battery voltage directly. In some embodiments, it is possible to provide a test current circuit having a voltage drop lower than battery voltage. A preferred embodiment can be thought of as altering the Figure 1 circuit by substituting a current sink for the current limiting resistor and transistor switch, and removing the boost supply. The current sink is preferably adequate to pass the test current from the current source, yet has a voltage drop of a fraction of a volt. Thus, the voltage of the test current circuit may be less than that of Figure 1 by the amount of the voltage drop across the resistor 28. The net voltage would be less than that of the vehicle battery so that a step-up regulator is not necessary. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a prior art deployment path resistance measurement circuit; Figure 2 is a schematic diagram of an embodiment of deployment path resistance measurement circuit; and Figure 3 is a schematic diagram of a current sink circuit used in the circuit of Figure 2. The ensuing description of an embodiment of circuit recites specific circuit values which are not critical. For example, the test current may be somewhat smaller than that suggested, so that in that case the current source and the current sink would both be adjusted accordingly. The Figure 2 circuit is based on the Figure 1 design with a few important changes, mainly that the current limiting device is a current sink rather than a resistor and there is no boost supply. Referring to Figure 2, the test current path between the battery 22 and ground comprises a diode 40, the constant current source 26, the external path 12, and a current sink 42, connected in series. The diode 40 provides reverse voltage protection in case the vehicle battery is connected with the wrong polarity. The current sink 42 is connected to line 38 for on/off control by the microprocessor. The voltage checking circuit comprising amplifier 32, A/D converter 34, and microprocessor 36 is the same as in the Figure 1 circuit. The constant current source 26 is also unchanged. The current sink 42 is designed to pass a slightly larger current than the rating of the current source 26. Then the voltage drop across the current sink will be small, 0.5 volts or less. The voltage drops on the other elements are approximately 0.7 volts for diode 40, 2 volts or more for the current source 26, and 0.75 volts for squib 14. Thus, the total resistance of the current test path provides a voltage drop of less than about 5 volts, well below the vehicle battery voltage range. The voltage drop of the current limiting device is of the same order of magnitude as the voltage across the squib or the external path. In the event that battery voltage is inadvertently applied to the external circuit 12, the current will be limited to the saturation current of the current sink 42 and the excess voltage will appear across the current sink. About 1.8 A is required to fire the squib; limiting the current through the external path 12 to about 10% of the firing current to provide the proper margin of safety. The current sink 42 should preferably have a capacity of 20 to 25 mA greater than the test current from the current source 26. A number of current sinks are known in the art which are appropriate for use in this application. For example, a current sink commonly used in integrated circuits is depicted in Figure 3. A "1X" transistor 44 and a "10X" transistor 46 have bases tied together and to the collector of transistor 44, while both emitters are grounded. The collector of transistor 46 is coupled to the test circuit to receive the test current. Transistor 46 has a base area 10 times that of transistor 44 so that its current will be 10 times greater than transistor 44. A constant current source 48 rated at 17 mA is supplied by the battery 22 and is controlled to turn on and off by the line 38 from the microprocessor. The 17 mA current is applied to transistor 44 so that the transistor 46 can conduct 170 mA at saturation. It will thus be seen that the above-described circuit can provide a test current for SIR external path diagnostics as well as over-voltage protection, while reducing the voltage requirement for the test circuit. The cost of the unit is reduced by eliminating a boost supply.

Claims: A diagnostic circuit for measuring the resistance of an external deployment path containing a squib (14) of a supplemental inflatable restraint system, comprising a voltage sensing and monitoring circuit (18) connectable across the external deployment path (12); and supply means (26,42) for supplying a predetermined current through the external deployment path and including a constant current source (26) connectable between a power source (22) and one side of the external path and a current limiting device (42) connectable in series with the constant current source (26) and the external path, the current limiting device (42) having a current limit less than that required to fire the squib; characterised in that the current limiting device (42) is a constant current sink (44,46) connectable between the other side of the external path and ground, the current sink (44, 46) being capable of passing a current greater than the predetermined current but substantially less than that capable of firing the squib. A diagnostic circuit according to claim 1, wherein the current sink (44, 46) is operative to generate a voltage drop of the order of the voltage drop in the external path. A diagnostic circuit according to claim 1 or 2, wherein the current sink (44, 46) produces in use a voltage drop less than substantially half a volt. A diagnostic circuit according to any preceding claim, wherein a diode (40) is coupleable between the constant current source (26) and the power source (22) for providing reverse voltage protection.

Inventors: KINCAID KEVIN DALE, RAVAS RICHARD JOSEPH JR, KINCAID, KEVIN DALE, RAVAS, RICHARD JOSEPH, JR.
Applicants: DELCO ELECTRONICS CORP, DELCO ELECTRONICS CORPORATION
Application year: 1994
Main classification: B60R21
IPC codes: B60R16, G01R31, B60R21, G01R27